GLASS-CERAMICS IN A COLD-CRUCIBLE MELTER : THE OPTIMUM COMBINATION FOR GREATER WASTE PROCESSING EFFICIENCY

摘要

Improving the efficiency of nuclear waste immobilization is constantly desired by all nuclear waste management programs world-wide.For high-level and other waste to be vitrified in traditional ceramic Joule-heated melters operated at temperatures up to 1150°C,process flexibilities including waste loadings are often restricted by this temperature limit as well as the need to consider wasteform corrosion of refractory linings and electrodes.New melter technologies,such as the cold-crucible melter (CCM),enable processing up to significantly higher temperatures free of many of the limitations of conventional melters.Higher processing temperatures open up the way for wider composition and processing envelopes to be considered for the vitrification process,including the possibility for higher waste loadings. In many instances the presence of crystals in the final cooled wasteform is not considered desirable within presently existing glass specifications.For some feed compositions increased waste loadings can lead to the formation of large amounts of crystals,and thus to a significant departure from the 'glass' state..Nevertheless it is recognized that,in general,increasing the acceptable volume fractions of crystals in the glass offers the best opportunity to increase waste loading,all other factors being equal.In addition,the deliberate promotion of specific crystalline phases by design may enhance the quality of the wasteform,for example by partitioning a long-lived radionuclide into a very stable crystalline phase,or by depleting the glass in detrimental elements.In order to explore the potential improvements by harnessing the higher achievable processing temperatures and immunity to refractory corrosion available with the cold-crucible melter,and after promising indications for synroc-based matrices,it was decided to investigate the feasibility of designing and producing via melting new high temperature 'glass-ceramic' wasteforms for high level waste immobilization. The INEEL calcines were selected as example feed compositions.These calcines have a wide range of problematic compositions.They either have high amounts of crystal-forming components,and/or components that lead to corrosive melts,and for good measure,the components in some waste types are quite refractory for vitrification as well.The recent DOE High-Level Waste Melter Review Report concluded that,for the INEEL calcine wastes in particular,the CCM could have sufficient advantages over the Joule-heated ceramic melter to justify its evaluation for direct vitrification of these wastes.Based on the extensive ceramic design experience of ANSTO,in collaboration with the CEA and COGEMA for a CCM implementation,a preliminary set of waste forms has been developed that immobilize long-lived waste actinides into highly chemically durable crystalline phases by design,using refractory crystal-forming components already in the wastes to advantage,while at the same time maintaining a very good overall leach resistance for the glass-ceramics even after "canister centerline cooling" ((CCC)heat treatments.This paper presents the results of a 50 kg technological scale test in the CCM of a glass-ceramic formulation for the average Bin Set 2 formulation,at a conservative waste loading of 50 %.